The horizontal, or spool, tree evolved from early through bore tree concepts as a means to retrieve production tubing and downhole equipment "through the bore" of the subsea tree in an effort to simplify equipment requirements and workover operations. Through bore trees were limited to single string completions and equipment of dimensions that could pass through seven-inch (178 mm) master and swab gate valves. Other penetrations, such as hydraulic stabs and electrical connectors, had to exit through side outlets in a tubing head below the tree.
With increasing demands for safety, operability, and cost; as well as the need to accommodate such equipment as electrical submersible pump (ESP) completions, the horizontal tree has developed to provide larger, multi-bore tubing hangers with additional penetrations to access and control down hole equipment. Due to its large bore configuration, the horizontal tree enables well control and vertical access to downhole equipment through a conventional marine drilling riser connected to a subsea blowout preventer (BOP), via a lower marine riser package (LMRP), eliminating the need for costly and specialized completion risers. The horizontal tree provides full downhole access without having to remove the tree or disturb any external connectors to flowlines, service lines, or central umbilicals.
A horizontal subsea tree generally comprises the same components found in conventional subsea trees such as tubing heads subsea valves, tubing hangers, and well head connectors. These components are, however, installed in a different order than in conventional trees. In a horizontal tree the wellhead is installed first, followed by the tree, the tubing hanger, and the tree cap, respectively. In a conventional tree the tubing hanger is installed before the tree. The order of installation in a horizontal tree eliminates a significant amount of intervention steps and equipment during workovers. It also eliminates certain equipment and reduces run time required for installation.
A key feature of the horizontal tree is the utilization of an internal plug fitted within the tubing hanger to redirect vertical flow through a horizontal side outlet. The tubing hanger has one or more horizontal side outlets for well bore and/or control fluids. After the tubing hanger is in place, a tubing plug is installed just above a side outlet to provide a first barrier and to redirect flow to that side outlet. Vertical access to the production bore is maintained once the plug has been removed to satisfy installation and workover requirements. In a conventional tubing hanger, a similar plug is used as a temporary pressure barrier during the installation process.
In accordance with National Rules and Regulations of some countries where subsea wellhead equipment may be operated, a double barrier must be maintained at all times to prevent leakage of well fluids. In a conventional tree, the barriers are comprised of the master and swab valves. In a horizontal tree, the barriers are comprised of removable plugs. For instance, one plug may be placed in the tubing hanger and the other, a full bore plug, placed in or over the top of the tree mandrel. Other arrangements are also possible.
In a horizontal tree the tree cap provides the second, full bore vertical pressure barrier behind the tubing hanger plug and the tubing hanger annulus seals. The tree cap may be one of two types: internal or external. Internal tree caps are run through the subsea BOP and drilling riser, establishing a second static barrier. External tree caps are installed with a separate running tool and work string after the BOP is disconnected and recovered. Their use may be limited, however, since during the operation in which they are run there is only a single barrier controlling a "live" well and the well may be required to be "killed". When the well is "killed", a heavy brine solution is introduced forming a hydrostatic head which serves as a first barrier.
Workover operations typically involving the use of wireline or coiled tubing tooling, require entry into the well through the christmas tree. Workover operations on a well with a conventional tree are carried out using a workover system comprising a safety package and a workover riser, which provides communication between the safety package and the pressure controlling equipment at the surface. The safety package includes a wire shearing valve and an emergency disconnect package (EDP). The workover system typically comprises a dual string assembly to provide communication with the production bore and the annulus. In a conventional tree it is required to install a workover system in place of the tree cap in order to accommodate entry into the well.
A horizontal tree does not require a dedicated workover system, but instead relies upon the use of a BOP stack and a subsea test tree (SSTT). The SSTT is run into the BOP stack on the high pressure tubing riser to which it is joined via a quick disconnect. The lower end of the SSTT is connected to the tubing hanger running tool, which may also be used to run and retrieve the full bore plug or tree cap. The BOP stack and the SSTT provide emergency shutoff sealing and disconnecting capability. The SSTT further provides an upper ball valve having wire shearing capability, and a lower ball valve.
Entry into the well in a conventional tree is through the barriers provided by the swab valve and the master valves. These valves can be opened to permit entry of tools into the well during a workover operation after the safety package has been installed. The barrier may be actuated selectively to open and close at any time while the safety package is in place. In the case of a horizontal tree, the well can be accessed only after the two barriers have been removed, rather than opened.
In the situation where a live, i.e. at full pressure, well is undergoing workover operations in which a workover system and wireline tool are installed, and an incident occurs which requires detachment of the rig from the well, emergency disconnection procedures are required. Under emergency conditions during workover operations on a live well fitted with a horizontal tree, it may be necessary for the surface vessel to be detached from the well quickly. In such a situation it may not be possible to re-install wireline plugs, which would comprise the well pressure containment barrier, immediately prior to disconnection. Under these circumstances, the only pressure containment barrier available is the BOP stack and SSTT.
The first step in emergency disconnection of a horizontal tree is to close the upper ball valve in the SSTT to shear the wireline, causing it to fall into the well. Such shearing may damage the valve and affect its pressure integrity. The lower ball valve in the SSTT is then closed to contain the well.
Next, the quick disconnect within the SSTT is separated to allow the high pressure tubing riser to be retrieved. Then the LMRP is separated from the BOP, leaving the BOP in place over the well. The barriers left in place are the lower SSTT valve and the BOP shear rams. In the event that the SSTT has become damaged during the disconnection and must be retrieved for repair, the only available barriers are the BOP rams. The damaged SSTT and associated tubing hanger running tool must be "stripped" through the BOP ram seals under pressure, considered by many well operators to be an unacceptably hazardous procedure. The BOP stack rams are employed to seal around the tubing hanger running tool during its disconnection from the tubing hanger and withdrawal from the well to enable the SSTT to be retrieved, and once the tool has been removed the BOP rams are used to shut in the well. In this case there are no metal sealing barriers in place to contain the well. Re-establishment of metallic sealing barriers cannot be achieved with the hanger running tool, SSTT and their associated tubing riser removed, as the tubing hanger plug and tree cap cannot be installed.
In order to reduce dependency on the BOP stack and to accommodate situations where the BOP stack or SSTT may not be present, it is desirable to provide a reliable, self-closing well pressure containment barrier using equipment normally located within the christmas tree.